We were at the local train museum yesterday. They have some old locomotives that are enormous. The wheels on them are as tall as I am. Why were they so large? Was it a casting problem or because of an inferior alloy that would overheat a smaller wheel? Or some other reason that escapes me?

I was also impressed with the engineering skills it took to design those behemoths.

The wheels on a steam locomotive were directly driven with no gearing. If you make the wheels too small, and as the train goes faster, you exceed how fast the engine can do its thing without coming apart, and you lose high end speed. Make the wheels too large, and the torque required to get the train moving from a stop becomes excessive if you are hauling a large, heavy load.

So the wheels that you see are a good balance between low-end torque and high-end speed.

Engines designed to haul heavy freight tended to have smaller wheels, where engines designed to haul passengers (higher speed, less weight) tended to have larger wheels.

Given your location I'm guessing you maybe got a peek at SP 4449, in red and orange Daylight colors. If so, its 80" drivers were just about as big as locomotive wheels ever got in the North American rail systems. There might have been a couple of 84" drivers somewhere, but I know what you mean about tall wheel sets when compared to the 40" and 42" wheels you see on big locos today.

The GNR Stirling class with the 8 ft 1 in drive wheels was probably the most extreme example. I think the drive wheels became smaller after that, as technology advanced and they could build pistons/cylinders that could reciprocate at a higher rate.

p.s. Turns out there's an even more extreme example - the Bristol and Exeter 4-2-4T with their 9-ft diameter drive wheels. ETA: Ninja'd!

By the way, it's exactly the same reason why early bicycles (penny-farthings) originally had large wheels. The legs can only turn the wheel so fast. The larger the wheel, the faster you go. (Though in case of the bicycle, an equally important factor was that a large wheel makes the ride smoother. The invention of the pneumatic tire and the chain drive removed both of these factors.)

The wheels on a steam locomotive were directly driven with no gearing. If you make the wheels too small, and as the train goes faster, you exceed how fast the engine can do its thing without coming apart, and you lose high end speed. Make the wheels too large, and the torque required to get the train moving from a stop becomes excessive if you are hauling a large, heavy load.

So the wheels that you see are a good balance between low-end torque and high-end speed.

Engines designed to haul heavy freight tended to have smaller wheels, where engines designed to haul passengers (higher speed, less weight) tended to have larger wheels.

Good stuff; thanks.

MonkeyMensch: yes indeed. Very impressive up close. We've ridden the Christmas train when they roll it out for a couple of weeks each year.

I was told by an old steam train engineer that the size of drivers determined the speed of the engine. A rough estimate of the top speed of a steam engine was found by taking the diameter of drive wheels and multiplying by 10. Gave you the speed by MPH. To go faster would shake the engine apart.

We were at the local train museum yesterday. They have some old locomotives that are enormous. The wheels on them are as tall as I am. Why were they so large? Was it a casting problem or because of an inferior alloy that would overheat a smaller wheel? Or some other reason that escapes me?

I was also impressed with the engineering skills it took to design those behemoths.

Additional factors for very early engines was the stess put by the wheel on the rail, and the slip when starting up.

Larger wheels have a larger contact patch, which meen they are less likely to break the rails.

If you watch the old film clips of steam engines starting up, you'll see the drive wheels slip. This is because the tourqe is not constant - it pulses. Faster engines with smaller wheels have faster pulses, which only works when the whole engine-wheel-rail system is better designed.

Conversely, larger wheels put the axle higher up, which is a bad thing.

The rule I heard is that he driver diameter in inches equals the running speed in miles per hour, which is close to what Snnipe 70E said. The top speed would be a bit higher, limited by the effective piston speed. You just can't get steam into and expanding and out of the cylinder fast enough beyond a certain point.
Some designs were limited in sustained speed by the boiler capacity, there was only so much steam available. Running beyond that point started to pull down the boiler pressure, essentially strangling the locomotive. Erie RR had two giant locomotives with three sets of driving wheels. Which meant three pairs of cylinders, etc, the third set of drivers was under the tender. The boiler couldn't mke enough steam to supply all that except at a very low speed and load. The Virginian RR had similar triplex locomotives, all of them (both rrs) had a short operating life.

a related question - the locomotive wheels had large solid pieces along the arc. are these to balance the wheel (seem too large for that) or did it provide some other benefit?

The piston rods and connecting rods (rods that connect the multiple wheels) are hanging from the wheels. The solid part of the wheel is on the opposite side, to balance the weight of the connecting rods and their supporting structure.

The higher the axle, the higher all the rest of the loco sits above the ground. Recognizing that rails are kinda close together, you get a big machine that's tall and narrow and has lots of weight up on top.

Which means it's tippy. All else equal you'd rather have a wider lower-slung machine to go faster around corners. Part of the reason train tracks have such wide curves is just avoiding tip-overs.

Specifically, the boiler needs to sit above the drive wheel axles. The boiler is the largest part of the locomotive. The drive wheels must be connected with solid axles - even if you could design one-sided wheel supports, it wouldn't work because the right/left cylinders will go out of sync.

Specifically, the boiler needs to sit above the drive wheel axles. The boiler is the largest part of the locomotive. The drive wheels must be connected with solid axles - even if you could design one-sided wheel supports, it wouldn't work because the right/left cylinders will go out of sync.

I think you could probably sync the cylinders. I think that the problem is that the train would drive off the rails.

The wheels are self-centering, but the self-centering is driven by the wheel at the other end of the axle.

You can feel this happening on a train. The characteristic sway of a train is synchronised to the self-centering action of the wheels on the track. If you go too fast for the train/wheel/track system, the train sways right off the track: this sets a maximum speed for a train, even on a straight level track.

Theoretically, with a super-large wheel design like the Hurricane, couldn't you hang the boiler under the axle?

And to the torque question, doesn't that also depend on how far out from the center the piston rods connect, too? It seems to me that, for any given piston rate, the speed would be determined by the ratio of those two radii.

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And to the torque question, doesn't that also depend on how far out from the center the piston rods connect, too? It seems to me that, for any given piston rate, the speed would be determined by the ratio of those two radii.

You're right about the ratios. But the stroke of the piston in the cylinder is equal to 2x the radius where the piston rod connects to the wheel.

Theoretically, with a super-large wheel design like the Hurricane, couldn't you hang the boiler under the axle?

As we were discussing in another thread, the distance between the rails is usually much less than the width of the locomotive. The space between the drive wheels is smaller still.

If you click on the 2nd of the links Bert Nobbins posted, you'll see another solution: the boiler is on a separate chassis from the pistons and drive wheels. This solves the issue of the locomotive being too tall, but you end up with much less weight on the drive wheels, which means inadequate traction.

Theoretically, with a super-large wheel design like the Hurricane, couldn't you hang the boiler under the axle?

One of the earliest designs of locomotive actually did have the boiler under the axles. The Novelty was one of the locomotives that competed at the Rainhill Trials, and had a boiler underneath the chassis, and the 'footplate' and firebox were both above the wheels. A neat little loco, but woefully underpowered. Novelty

One of the builder/designers of Novelty went on to design the USS Monitor, one of the first steam-driven ironclads.

In the early 1980's I spent about 6 months in India riding the old steam locomotives from place to place.

Watching them steam into the train stations was one of the highlights of the trip. Even at that time they were being phased out. Sadly they are probably all but gone today.

Genuine, everyday steam working on India's railways came to an end -- after many years' gradual fading-out -- approximately in the year 2000. In a few restricted locations in India, steam locos continue to be used; but consciously and deliberately as tourist- and railfan-bait -- which takes most of the shine off it for me.

Regrettably for folk such as myself, there is now almost nowhere left on earth where steam locos are in everyday, commercial use; as opposed to in "entertainment" roles of one or another kind.

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